I don't get it...is it me?

Why is it so hard for a guy to stay faithful? I been with my boyfriend for 8 months and it was going great. He treated me like I was the best thing in the world. Then one night i looked in his phone and what do I find text messages between him and some girl and him asking is she gonna give him head. Now she knows he has a girlfriend but guess what she says...she says yeah. I been thinking maybe its me...maybe its something wrong with me